Rust has the clap_complete package for its most popular arg parsing library: https://crates.io/crates/clap_complete

ripgrep exposes its (bespoke) shell completion and man page generation through a --generate option: rg --generate=man, rg --generate=complete-bash, etcetera. In xh (clap-based) we provide the same but AFAIK we're the only one to copy that interface.

Symfony (for PHP) provides some kind of runtime completion generation but I don't know the details.